Conventionally known automatic transmission reduces malfunctions such as excessive rises in the pressure of engagement hydraulic chambers of pistons, which engage or disengage clutches (i.e., frictional engagement elements), over set values. For this purpose, each centrifugal balance chamber for cancelling the centrifugal hydraulic pressure is provided on the opposite side of the engagement hydraulic chamber with a piston interposed therebetween. The centrifugal balance chamber stores hydraulic oil. As a result, not only in the engagement hydraulic chamber but also in the centrifugal balance chamber, the same level of centrifugal hydraulic pressure is generated so that the centrifugal hydraulic pressure generated in the engagement hydraulic chamber and the centrifugal hydraulic pressure generated in in the centrifugal balance chamber cancel one another.
In this automatic transmission, there is a need to discharge excessive hydraulic oil from the centrifugal balance chamber. For example, Patent Document 1 shows a drum clutch of automatic transmission, in which a long hole (i.e., through-hole) for discharging hydraulic oil outside is formed in a centrifugal cancel plate (i.e., seal plate) for providing a centrifugal chancel chamber (i.e., centrifugal balance chamber) between a clutch piston and the centrifugal chancel chamber.